Combustion or gas turbines are fueled by a pressurized combustible product, such as propane, natural gas, kerosene or jet fuel that supplies energy to drive a plurality of concentric turbine blades mounted on a common shaft. The air is first compressed in a compressor stage then directed to the combustion area. Here the fuel mixes with air and burns, with the resulting high-pressure high-velocity gas directed to the turbine blades, rotating the turbine to extract energy from the combustion process. The turbine shaft is axially connected to a generator shaft, and thus rotation of the turbine shaft imparts rotational energy to the generator shaft for producing electricity as will be described further below. Combustion turbine/generator combinations are available with selectable power output ratings and thus can be used to supply power to an industrial facility or to the electrical grid. Gas turbines are also used as aircraft engines.
It is known that gas turbines are not self-starting machines. Typically a separate motor is employed to provide the starting rotational energy until the gas turbine reaches an operational rotational speed, at which time the motor is disconnected and the combustion process supplies the energy to drive the turbine. For example, the motor drives the turbine up to a speed of several hundred revolutions per minute (rpm), at which point the combustion turbine is ignited. The motor continues to supply rotational assist as the generator reaches its self-sustaining speed. At this point the motor is disconnected since the gas turbine can provide sufficient energy to allow the generator to develop the rated electrical output.
Because of its size and considerable weight, the rotating turbine shaft of large combustion turbines is susceptible to bowing if it remains in one position for an extended period. To overcome this problem, a turning motor (also referred to as a turning gear because the motor output is supplied to the turbine shaft via a gear box) is provided for turning the shaft at a slow speed (for example, 3 rpm) when the unit is not operating. The turning motor also prevents binding of the turbine shaft that can be caused by uneven shaft cooling as the shaft rotational speed declines.
Known generator action of a dynamoelectric machine is employed in the generator to produce the electrical output. Conventionally, a generator comprises at least one stator winding and a rotating field winding. Current is supplied to the field winding from an exciter, as described further below, for inducing current flow in the stator windings as the magnetic field of the rotating field winding cuts across the stator windings. Three-phase alternating current output is supplied from a generator having three independent stator windings spaced at 120° intervals around the stator shell. Single phase AC is supplied from a single stator winding.
The exciter provides direct current (DC) to the rotating field winding of the main generator. Like the main generator, the exciter employs generator action to develop the DC output. The exciter DC output is also regulated to control the intensity of the magnetic field developed by the main generator field winding. Since the stator winding is responsive to this magnetic field, the main generator AC output is thus controlled by the DC input to the rotating field winding.
In one embodiment, the exciter utilizes a rotating winding on the same turbine-driven shaft as the main generator, and a stationary field winding responsive to an externally-generated DC current. As the rotating winding rotates through the stationary magnetic field of the field winding, an AC current is induced in the former. The AC is converted to DC by a rectifier bridge mounted on the rotating shaft, and the resulting DC current is supplied to the main generator field winding through conductors also mounted on the rotating shaft.
Static start, in which the generator operates as a motor, is another known technique for starting a gas turbine engine. Alternating current is supplied to the generator stator winding and alternating current is supplied to the rotating armature from a separate static-start exciter. The AC current is rectified by operation of the rotating rectifier to supply direct current to the generator field. The motoring action produced by the interacting magnetic fields drives the generator, and in turn drives the turbine. Start-up control of the turbine is exercised as described above in conjunction with the separate start-up motor embodiment. Specifically, when the generator/motor speed reaches a pre-determined speed (between 50% and 85% of rated speed), the motoring action is terminated by opening breakers supplying current to the field and armature windings and closing breakers supplying exciting current from the exciter to the rotating field winding to produce generating action.
FIG. 1 illustrates a prior art exciter 10 for supplying DC current to a main generator 12 driven by a primer mover 14, such as a gas turbine, by way of a shaft 15. The generator 12 comprises a field winding 16 responsive to a DC current for producing a three phase AC current in a stator winding 18.
DC current is supplied from an external source to an exciter field winding 26. In response to the DC current, an AC current is induced in the exciter armature winding 28, also driven by the shaft 15. The AC current is rectified by a diode bridge 30 and the output DC current is thus supplied to the field winding 16, which in turn produces the generator output power.